Generally there have been two kinds of catcher-charge plate combinations in ink jet printing head relevant to a consideration of this invention. In one, the catcher has not been integral with the charge plate, but rather has been separated therefrom by an air gap. In this arrangement, the uppermost edge of the catcher that is closest to the oncoming jets or stream of drops has been horizontally set back from the operative charging surface of the charge plate, a variety of distances. Such a design is shown, e.g., in U.S. Pat. No. 4,107,699, FIG. 1.
In the other, the catcher and the charge plate are integral, providing certain advantages. One is that alignment of the two parts occurs automatically, so that there are no realignment adjustments when either part has to be replaced. A single heater can also be used to heat both parts, since they are integral. In such a design, there generally has been substantially no set back whatsoever, as described for the first design, of the uppermost edge of the catcher closest to the oncoming jets, relative to the operative charging surface of the charge plate. Such a design, although useful, has been found to have some drawbacks. For example, the lack of any setback tends to encourage arcing between the charge plate and the catcher, to the electrolytic detriment of both, and particularly the individual leads of the charge plate. In addition, the intercept face of the catcher has a negative slope, defined hereinafter. A negative slope tends to provide a catcher face that is parallel to the drops to be caught, thus necessitating a longer catching surface than would otherwise be the case. Such longer surface is disadvantageous both because of the additional distance the drops must travel to the paper being printed and because the collected ink film thereon is more susceptible to wide variations in thickness, than is a shorter length of collected film. That is, the longer catching surface produces a rapidly thinner coat as caught ink flow drops off when more ink is being printed. The thinning can cause drops intended to be caught, to be missed. Conversely, as the flow of caught drops increases, the volume of the film of ink tends to bulge too much, due to the increased length of catch surface, and interfere with print drops that are supposed to miss the catcher.